Level measurement systems, i.e. for use in measuring the level of a material within a vessel, are well known. It is also known to locate the boundaries between different materials in a vessel by measuring the density of the vessel contents at different locations to form a density profile of the vessel and contents in order to identify density changes which are indicative of boundary regions. For example, it has been proposed in WO 00/22387 to measure the density profile of a medium by providing an axially distributed array of sources of gamma radiation to give collimated beams of said radiation and an axially distributed array of detectors disposed so that the medium under study extends between the sources and the detectors. By monitoring the radiation received by the detectors, the amount of radiation absorbed by the medium from each beam can be determined and so variations in the medium density can be detected. One disadvantage of the system of WO 00/22387 is the need to employ radioactive materials which consequently imposes health and safety issues in order to ensure safe working.
GB 1524303 describes an apparatus used to determine the boundary between liquids of different densities by means of an elongated member along the length of which a series of housings is disposed with spaces between each housing into which spaces the medium under study can enter through perforations in the wall of the elongated member. Each housing comprises an ultrasonic transmitter and detector and the end of each housing acts as a reflector to reflect waves from the transmitter of the next adjacent housing back, through the medium in the space between the housings, to that next adjacent housing. One disadvantage of this type of arrangement is that the vertical resolution is limited by the need to provide a vertical space between each of the housings.
WO03/012379 describes apparatus for monitoring the composition of a medium comprising an elongated member for insertion into said medium, said member having an array of transmitters and receivers of ultrasonic waves disposed at intervals along at least part of its length, and reflector means to reflect transmitted ultrasonic waves to the receivers, said reflector means being supported by, but laterally spaced from, said member whereby, when said elongated member is inserted into said medium, the medium occupies the space between said elongated member and said reflector means, and said transmitters, receivers and reflector means being disposed so that transmitted ultrasonic waves pass through said medium en route to the receivers, and monitoring means providing a signal dependent upon the time taken for an ultrasonic sound wave to travel from a transmitter to a receiver associated therewith.
U.S. Pat. No. 4,565,088 describes an apparatus for the detection of changes of composition in a desalting vessel in which an ultrasonic probe emits sound towards a mirror which is arranged to travel vertically within a dip tube. The mirror directs the sonic energy out of the dip tube towards a reflector positioned apart from the dip tube within the vessel so that the energy is reflected back to the mirror and up the dip tube to be received by the probe.